1. Technical Field of the Invention
The present invention relates to a vehicle control apparatus that performs idle stop control (hereinafter referred to as “IS control”) under which an engine (internal combustion engine), i.e. a drive source of travel, is stopped when predetermined conditions are met to perform idle stop.
2. Related Art
In a braking system, a pedal pressure, which is a pressure caused when a brake pedal is pressed by a driver, is intensified by a servomechanism to cause high pressure in a master cylinder (this pressure is hereinafter referred to as “M/C pressure”). The M/C pressure is then transferred to each wheel cylinder to cause high pressure in the wheel cylinder as well (this pressure is hereinafter referred to as “W/C pressure”) for the generation of high braking force. A brake booster of an intensification-with-negative pressure type may correspond to such a servomechanism for intensifying a pedal pressure, such a brake booster uses the negative pressure caused by an intake stroke of an engine. The negative pressure is introduced into the brake booster (this pressure introduced into the brake booster is hereinafter referred to as “vacuum pressure”) and used for intensifying the pedal pressure.
Meanwhile, a control system for automatically stopping and restarting an engine has been known. This control system is adapted to automatically stop an engine when the driver makes an attempt to stop the vehicle, thereby improving fuel consumption. As mentioned above, a brake booster uses the negative pressure caused by an intake stroke of an engine. When such a brake booster is used in the control system for automatically stopping and restarting an engine, the negative pressure for the booster is decreased when the engine is stopped, resulting in decrease of the braking force.
In order to suppress decrease of braking force, JP-A-2003-013768 suggests a technique in which an engine is restarted as the vacuum pressure is decreased. Specifically, in this technique, combinations of a vacuum pressure and a braking operation are stored in advance. When it is estimated that a combination of a negative pressure and a braking operation is likely to decrease the vacuum pressure, the engine is ensured to be restarted. Restart of the engine can again guarantee the vacuum pressure to thereby suppress decrease of the braking force.
However, an engine may be stopped under the IS control not only when the vehicle is stopped but also when the vehicle is decelerated. In the latter case, since the vehicle is still moving, the driver may attempt to adjust the vehicle speed to a desired speed depending on the conditions of the road surface, such as of a downward slope, on which the vehicle travels. Therefore, the driver may frequently operate the brake before the vehicle is stopped. The driver's frequent brake operation may decrease the vacuum pressure.
In such a case, the engine may be restarted by detecting the decrease of the vacuum pressure by detecting the fact that the vacuum pressure has become equal to or lower than a threshold. Alternatively, the engine may be restarted by predicting the decrease of the vacuum pressure. However, restart of an engine in these manners may invite a response delay. Specifically, it may take time from when the engine is restarted until when the vacuum pressure is guaranteed, leading to difficulty in suppressing decrease of braking force.
Another measure that may be taken against the decrease of the vacuum pressure may be to restart the engine at an earlier stage in preparation for the response delay. In this case, however, the frequency of performing idle stop is lowered and thus the effect of improving fuel consumption will not be well exerted.
Further, when an engine is restarted by a starter, it sometimes happens that the engine may not be normally restarted (e.g., restart of the engine may be delayed). In this case, vacuum pressure is not generated for intensifying the pedal pressure. As a result, the braking force may no longer be suppressed from being decreased.